Composite material and method for the production thereof

ABSTRACT

A composite material with a porous inorganic-nonmetallic matrix and a second material, is characterized in that the porous inorganic-nonmetallic matrix has a bending strength of ≧40 MPa as measured according to ISO 6 872; the second material is an organic material which at least partly fills the pores of the porous matrix; and the composite material has a modulus of elasticity, E, of ≧25 GPa as measured according to ISO 10 477.

This is a nationalization of PCT/EP02/02567 filed Mar. 8, 2002 andpublished in German.

The present invention relates to a composite material with a porousinorganic-nonmetallic network whose pores are filled with a polymer, amethod for the preparation thereof, and applications of this compositematerial.

EP-A-1 006 095 relates to a method for the preparation of endodontic,orthodontic and direct restorations based on a ceramic network. Astarting glass monomer is filled with a material to achieve improvedflexibility and abrasion resistance. The infiltration material may be,for example, a glass which will form a layer having a hardness within arange of from 300 to 600 KHN and a modulus of elasticity within a rangeof from 70 to 80 GPa. Its bending strength is within a range of from 200to 500 MPa. In the interior, the structure is to have a bending strengthof about 150 to 80 MPa and a modulus of elasticity of from 15 to 25 GPa.The corresponding values are to be achieved by infiltration with amonomer and subsequent curing. Thus, a two-layered structure has beendescribed which has different properties. Corresponding products havenot been known to date.

EP-A-0 803 241 relates to a dental material made of a porous ceramicmaterial which is filled into an artificial tooth, an inlay, an onlay, acrown, a crown bridge or within a block which is suitable for CAD/CAMprocessing. This material is sintered to form interconnected pores. Thisis followed by impregnation with an artificial resin. A drawback ofthese described products is the inaccurate definition and description ofthe supporting network. In many embodiments, this leads to adeterioration of the physical-technical data in comparison to compositematerials common today. U.S. Pat. No. 5,843,348 relates to a method forthe preparation of a ceramic network material from a ceramic suspension.This method involves the casting of a suspension into a mold which has atooth-shaped design. The suspension contains dispersed aluminum oxideparticles in a medium which contains deionized water with a pH within arange of from 4 to 5 and polyvinyl alcohol in a concentration of from0.5 to 1% by weight. After drying, the suspension is fired in an oven ata temperature within a range of from 1000 to 1400° C. to produce aceramic network. The porous structure is infused with lanthanumaluminosilicate glass to form a glass layer having a thickness within arange of from 1 to 2 mm within the ceramic network.

In Ullmann's Encyclopedia of Industrial Chemistry, different methods aredescribed for the preservation of porous rock structures. Thus, theobjects, for example, sculptures of sandstone or marble, are impregnatedwith hydrophobic reagents, such as alkyltrialkoxysilanes, and/orcoupling agents, such as tetraethyl orthosilicate. For the stabilizationof the structure, the pores are additionally infiltrated with monomers,for example, methyl methacrylate, under vacuum. After thermal curingunder pressure, the objects are mechanically more stable, and moreimportantly, chemical decay is substantially prevented by closing thepores.

It is the object of the invention to provide a composite material whichhas a homogeneous poreless structure and properties which are betweenthose of ceramic and conventionally filled polymer materials, and toprovide a method for the preparation of such composite material.

This object is achieved by a composite material with a porousinorganic-nonmetallic matrix and a second material, characterized inthat

-   -   said porous inorganic-nonmetallic matrix has a bending strength        of ≧40 MPa as measured according to ISO 6 872;    -   said second material is an organic material which at least        partly fills the pores of said porous matrix; and    -   said composite material has a modulus of elasticity, E, of ≧25        GPa as measured according to ISO 10 477.

The composite material according to the invention is advantageousbecause a new class of materials is obtained whose properties arebetween those of ceramic and plastic materials. For example, this classof materials is characterized, on the one hand, by a lower brittlenessas compared to ceramic materials and, on the other hand, by an increasedabrasion resistance as compared to the previous inorganic-filledpolymers.

The inorganic-nonmetallic phase of the composite material according tothe invention forms a network which has an intrinsic strength (bendingstrength of the porous inorganic-nonmetallic network prior toinfiltration), σ, of ≧40 MPa. This results in an increase of the modulusof elasticity of the composite material as compared to conventionallyfilled composites.

The composite material according to the invention preferably has abending strength, σ, of ≧100 MPa as measured according to ISO 6 872.

The pores of the porous network occupy a volume of from 5 to 85% byvolume, preferably from 10 to 50% by volume, especially from 15 to 35%by volume. An advantage of the controllable porosity of theinorganic-nonmetallic network is the fact that the polymer content inthe later composite material correlates therewith. An increase of thepolymer content is related with an increase of the impact strength and adecrease of the specific gravity of the composite material.

Typically, the pores of the inorganic-nonmetallic network of thecomposite material according to the invention have a pore size of from0.2 to 25 μm, preferably from 0.5 to 10 μm. The pore sizes and shapes tobe employed are dependent on the combination of inorganic and polymermaterials, on the contact angle between the inorganic material and themonomer or polymer, on the pretreatment and on the infiltration methodemployed.

The porous inorganic-nonmetallic network as an intermediate stage of thecomposite material according to the invention can be obtained, forexample, by the sintering of powdery inorganic, especially ceramic,substances or mixtures of substances, the sintering process beingcompleted, in particular, prior to the formation of closed pores.Further, it is preferred that the sintering process is discontinued onlyafter the phase of necking or glass flow, which is typically associatedwith a sudden change of the intrinsic strength of theinorganic-nonmetallic network.

Preferably, the powdery inorganic substances or mixtures of substancesemployed have grain sizes which exhibit a bimodal distribution, e.g.,fine and coarse grain sizes. The small grains have a higher sinteringactivity while the large grains determine the pore shape.

In a preferred embodiment, the inorganic-nonmetallic network of thecomposite material according to the invention comprises powderyinorganic substances or mixtures of substances which have a grain sizeof from 0.2 μm to 25 μm. Typical d₅₀ values (laser granulometry) of thestarting materials employed are between 0.5 and 5 μm.

The inorganic-nonmetallic network of the composite material according tothe invention is preferably constituted of at least two different powdermixtures having different sintering temperatures. The sintering activityis determined by the lower-melting powder components.

The pores of the inorganic-nonmetallic network advantageously havesurfaces which possess hydrophobic properties. The hydrophobicproperties can be achieved, for example, by superficial silanization.The hydrophobic surfaces increase the wettability by monomers of theporous network.

The silanization is effected simply by means of a silanizing agent in aliquid phase.

For the silanization of the porous inorganic-nonmetallic network, analkoxysilane or halosilane, preferably3-methacryloxypropyltrimethoxysilane, is employed.

In particular, the inorganic phase of the composite material accordingto the invention is an organic polymer which is formed in situ withinthe pores of the porous network by the polymerization of prepolymers,oligomers or monomers.

The organic polymer is formed from thermally polymerizable monomersand/or by chemically induced initiation reactions of polymerizablemonomers and/or by condensable monomers.

The composite material according to the invention is preferablyisotropic. This eliminates the disadvantages of today's anisotropiccomposites which have, for example, only low strengths when stressedagainst the preferential direction. Nevertheless, it can be imaginedthat some well-aimed anisotropy may be introduced into the compositematerial according to the invention. This can be effected, for example,by introducing fibers which are selectively cross-linked in otherdirections of space. Further, it is possible to prepare continuousgradient materials by using gradient ovens in the preparation of theinorganic-nonmetallic network.

The composite material according to the invention may contain auxiliaryagents, such as antioxidants and pigments which are appropriate to therespective intended application.

The composite material according to the invention can be prepared, forexample, by a method as follows:

-   -   preparing the inorganic-nonmetallic starting material;    -   shaping the inorganic-nonmetallic phase, either wet, for        example, by a slip process, or dry, for example, by isostatic        compression, optionally using a suitable binder system;    -   sintering the inorganic-nonmetallic network to the desired        degree of sintering and porosity;    -   the sintering process being discontinued before substantially        closed pores are formed in the sintered product;    -   and after the phase of necking and/or glass flow has been        reached;    -   the porous product of inorganic-nonmetallic material obtained is        first coated with a wetting agent, preferably silanes having a        suitable functional group;    -   the inorganic-nonmetallic network is then completely infiltrated        with monomers;    -   and subsequently polymerized with a suitable method, such as hot        polymerization or microwaves.

In the method according to the invention, oxide ceramics, glasses,porcelains, non-oxide ceramics and combinations thereof, for example,are employed as the inorganic material.

In the method according to the invention, powdery inorganic materialshaving a grain size of from 0.2 μm to 25 μm, preferably from 0.5 to 10μm (d₅₀ values as determined by laser granulometry), are employed, inparticular.

Preferably, according to the invention, the inorganic material in aliquid form is infiltrated into the sintered inorganic material.

The wetting agent is preferably in solution. An advantage of thedilution is the decreased viscosity.

The wetting agent must contain a functional group capable of coupling.

According to the invention, organic monomers or prepolymers areintroduced into the sintered inorganic-nonmetallic network andpolymerized within the pores of the network to form the organicmaterial.

According to the invention, the organic material can be introduced intothe sintered inorganic material by pressure infiltration. The advantagethereof is the quickly achieved complete and homogeneous infiltration.Depending on the objective, it may be found advantageous to perform thepolymerization under pressure.

Optionally, both the inorganic-nonmetallic network and the organicmonomer may be evacuated prior to infiltration.

As the monomers, organic compounds are preferably employed which have atleast one ethylenically unsaturated moiety, at least one condensablemoiety or at least one moiety capable of ring-opening polymerization, orcombinations thereof.

Suitable initiator systems for the polymerization are known and can beseen from the relevant literature.

For the preparation of translucent materials suitable for dentalpurposes, according to the invention, feldspar-containing powders andfrits are employed as the inorganic material, and bismethacrylates areemployed as the organic compound, and peroxide-containing compounds areemployed as initiators.

The invention also relates to a compound material for dentalapplications which can be obtained by the method according to theinvention.

The compound material according to the invention can be preferablyemployed for dental purposes, such as for inlays, onlays, crowns andbridges.

Compound materials prepared according to the invention can be used,inter alia, for the surface coating of ceramic and metallic materials,composites and plastic materials, as component parts having novelproperties, such as modulus of elasticity, abrasion properties, specificgravity, heat distortion resistance. Further embodiments aresound-insulating and heat-insulating elements, friction bearings,members for vibration damping, electric isolators etc.

In particular, the composite material according to the invention can beemployed as a friction bearing, for heat and/or sound insulation, or asa vibration damper.

The porous inorganic nonmetallic matrix having a bending strength of ≧40MPa as measured according to ISO 6 872 which can be employed for thepreparation of the composite material according to the invention mayserve as an intermediate product, and the invention also relates to suchintermediate.

Preferably, the porous inorganic nonmetallic matrix according to theinvention is formed from oxide ceramics, glasses, porcelains, non-oxideceramics or combinations thereof.

In particular, the pores of the porous inorganic nonmetallic matrixaccording to the invention occupy a volume of from 5% by volume to 85%by volume, preferably from 10 to 50% by volume, especially from 15 to35% by volume.

Also claimed according to the invention is a method for the preparationof a porous inorganic-nonmetallic matrix according to the invention;

-   -   wherein an inorganic nonmetallic material is mixed with a        removable binder to form a moldable material;    -   the binder is removed to obtain a porous inorganic nonmetallic        structure;    -   the porous inorganic nonmetallic structure is sintered;    -   to form said porous inorganic nonmetallic matrix.

The invention also relates to a method for the preparation ofimpressions from objects using the porous inorganic-nonmetallic matrixaccording to the invention. This method according to the invention ischaracterized in that:

-   -   an inorganic-nonmetallic material is mixed with a removable        binder to form a moldable material;    -   said moldable material is contacted with an object from which an        impression is to be prepared so that an impression of the object        in the form of a negative is formed in the moldable material;    -   the moldable material is detached to obtain the shape of the        object from which the impression is to be prepared, followed by        removing the binder;    -   optionally followed by sintering and infiltrating the structure        obtained after the removal of the binder.

In a further embodiment of the method mentioned, a method for thereplication of objects is obtained. In this method, the method accordingto the invention for the preparation of impressions from objects isfirst performed. The matrix obtained thereby is itself subjected to thepreparation of an impression to obtain a replicate of the object to bereplicated. However, the method may also start from an impression of anobject obtained in some other way to obtain a negative form from whichan impression is then prepared according to the impression methodaccording to the invention and optionally solidified.

Also claimed according to the invention is a mixture of powderyinorganic nonmetallic substances or mixtures of substances and aremovable binder. This mixture is characterized in that said powderyinorganic nonmetallic substances or mixtures of substances have a grainsize, d₅₀, of from 0.2 μm to 25 μm, and the binder is present in anamount of from 2% by weight to 50% by weight, based on the total weightof the mixture.

The powdery inorganic nonmetallic substances or mixtures of substancesare preferably oxide ceramics, glasses, porcelains, non-oxide ceramicsor combinations thereof.

Preferably, powdery inorganic substances or mixtures of substances withgrain sizes having a bimodal distribution are employed.

In particular, the powdery inorganic nonmetallic substances or mixturesof substances for the preparation of the paste according to theinvention have a grain size, d₅₀, of from 0.5 μm to 5 μm as measured bylaser granulometry.

In the mixture according to the invention for the preparation of thepaste, at least two different powders and/or mixtures of powders, havingdifferent sintering temperatures may be present. The paste according tothe invention contains additives which allow a well-aimed curingaccording to the prior art.

The porous inorganic-nonmetallic matrix according to the invention canbe used for obtaining molded parts which have advantageous properties;the invention also relates to such molded parts. The molded partsaccording to the invention can be prepared from porous sintered naturaland/or synthetic feldspars or feldspatoids having a porosity whichcorresponds to a volume of from 5% by volume to 85% by volume,preferably from 10 to 50% by volume, especially from 15 to 35% byvolume. By infiltration with suitable monomers and subsequentpolymerization, transparent materials suitable for dental applications(e.g., use as inlays, onlays, veneers, crowns or bridges) can beprepared.

Particularly advantageous are multilayered molded parts having severallayers with different properties in the layers, obtainable by coatingdifferent pastes with different resulting inorganic-nonmetallic matrixcomponents, followed by sintering. The preparation of the completecomposite material is then effected as previously described.

Molded parts having continuously changing properties can be preparedaccording to the invention, and the invention also relates to suchmolded parts. These molded parts can be obtained, for example, bysintering the inorganic-nonmetallic matrix in a gradient oven. Naturalteeth are also anisotropic and comprised of several different layers.Typically, the molded parts according to the invention are prepared asfollows:

-   -   preparing the work with the mouth, applying a release agent;    -   for cavities, direct preparation of an impression within the        mouth using the paste according to the invention;    -   preliminary or final curing of the molded part, removing it from        the cavity;    -   sintering to form pores with firing out the binder.    -   for crowns and bridges: preparing an impression from the        preparation;    -   preparing the master;    -   building the restoration on the master using paste according to        the invention;    -   sintering the work to form pores with firing out the binder.

The invention will be further illustrated using the following Examples.

EXAMPLE 1

A bimodal aluminum oxide having a d₅₀ of about 2.5 μm was pasted withdistilled water and usual additives (in this case citric acid andDarvan) and ultrasonic treatment to form a suspension which is suitableas a slip. With this suspension, Delrin molds having dimensions of1.2×4×20 mm were cast. After drying, the parts were taken from the moldand fired at a peak temperature of 1120° C. and a holding time of 2hours.

The fired porous parts were then soaked with a 5% solution ofmethacryloxypropyltrimethoxysilane and again dried. Thereafter, theparts were evacuated and infiltrated over night at room temperature withan evacuated 1:1 mixture of urethane dimethacrylate and triethyleneglycol dimethacrylate, followed by polymerization over a period of 17hours, the peak temperature being 80° C.

On a Zwick universal testing machine, the parts exhibited an averagebending strength of 300.15 MPa and a modulus of elasticity of 76.23 GPa.

EXAMPLE 2

A bimodal magnesium aluminum oxide spinel having a d₅₀ of about 2.5 μmwas pasted with distilled water and usual additives (in this case citricacid and Darvan) and ultrasonic treatment to form a suspension which issuitable as a slip. With this suspension, Delrin molds having dimensionsof 1.2×4×20 mm were cast. After drying, the parts were taken from themold and fired at a peak temperature of 1180° C. and a holding time of 2hours.

The fired porous parts were then soaked with a 5% solution ofmethacryloxypropyltrimethoxysilane and again dried. Thereafter, theparts were evacuated and infiltrated over night at room temperature withan evacuated 1:1 mixture of urethane dimethacrylate and triethyleneglycol dimethacrylate, followed by polymerization over a period of 17hours, the peak temperature being 80° C.

On a Zwick universal testing machine, the parts exhibited an averagebending strength of 256.87 MPa and a modulus of elasticity of 82.89 GPa.

EXAMPLE 3

A bimodal mixture of 67% aluminum oxide and 33% zirconium dioxide havinga d₅₀ of about 2.5 μm was pasted with distilled water and usualadditives (in this case citric acid and Darvan) and ultrasonic treatmentto form a suspension which is suitable as a slip. With this suspension,Delrin molds having dimensions of 1.2×4×20 mm were cast. After drying,the parts were taken from the mold and fired at a peak temperature of1180° C. and a holding time of 2 hours.

The fired porous parts were then soaked with a 5% solution ofmethacryloxypropyltrimethoxysilane and again dried. Thereafter, theparts were evacuated and infiltrated over night at room temperature withan evacuated 1:1 mixture of urethane dimethacrylate and triethyleneglycol dimethacrylate, followed by polymerization over a period of 17hours, the peak temperature being 80° C.

On a Zwick universal testing machine, the parts exhibited an averagebending strength of 287.42 MPa and a modulus of elasticity of 79.12 GPa.

EXAMPLE 4

A bimodal mixture of two feldspar frits (frit 1: firing temperatureabout 830° C., 10% proportion in the mixture; frit 2: firing temperatureabout 1180° C., 90% proportion in the mixture) having a d₅₀ of about 4.5μm was pasted with a usual modeling liquid (water+binder additive) toform a suspension which is suitable as a slip. This suspension wasshaken into metal molds having dimensions of 25×5×1.6 mm. After drying,the parts were taken from the mold and fired at a peak temperature of940° C. and a holding time of about 40 minutes.

The fired porous parts were then soaked with a 5% solution ofmethacryloxypropyltrimethoxysilane and again dried. Thereafter, theparts were evacuated and infiltrated over night at room temperature withan evacuated 1:1 mixture of urethane dimethacrylate and triethyleneglycol dimethacrylate, followed by polymerization over a period of 17hours, the peak temperature being 80° C.

On a Zwick universal testing machine, the parts exhibited an averagebending strength of 148.83 MPa and a modulus of elasticity of 30.04 GPa.

The parts exhibit an excellent translucency and are suitable foresthetic dental restorations due to their optical properties.

1. An isotropic composite material comprising a porousinorganic-nonmetallic matrix having a bending strength greater than orequal to 40 MPa as measured according to ISO 6 872 and an organicmaterial at least partly filling the pores of the porousinorganic-nonmetallic matrix, such that the isotropic composite materialhas (i) a modulus of elasticity greater than or equal to 25 GPa asmeasured according to ISO 10 477 and (ii) a bending strength greaterthan or equal to 100 MPa as measured according to ISO 6
 872. 2. Thecomposite material according to claim 1, characterized in that theinorganic-nonmetallic matrix comprises a sintered powdery inorganicsubstance or mixture thereof, the powdery inorganic substance having abimodal distribution grain size.
 3. The composite material according toclaim 2, characterized in that the grain size d₅₀ is 0.2-25 μm asmeasured by laser granulometry.
 4. The composite material according toclaim 2, characterized in that the grain size d₅₀ is 0.5-5 μm asmeasured by laser granulometry.
 5. The composite material according toclaim 1, characterized in that the inorganic-nonmetallic matrixcomprises a sintered mixture of at least two powdery inorganicsubstances having different sintering temperatures.
 6. The compositematerial according to claim 1, characterized in that theinorganic-nonmetallic matrix has pore surfaces coated with a couplingagent rendering the pore surfaces hydrophobized and provided withfunctional groups.
 7. The composite material according to claim 1further comprising silanized pore surfaces of the inorganic-nonmetallicmatrix characterized in that the silanizing agent isaminopropyltriethoxysilane, vinyltriethoxysilane,3-methacryloxypropyltrimethoxysilane, or a mixture thereof.